In a cellular communications system, subscriber units communicate voice and/or data with one or more base stations. As a subscriber unit moves through a coverage area, the subscriber unit communication channel is switched or handed off from one base station to another base station in order to improve the communication link.
In more sophisticated cellular communication systems, the subscriber unit takes on a greater share of the task of deciding which base station to communicate with. For example, when the subscriber unit is in an idle mode, which is a mode in which the subscriber unit has not been assigned a traffic channel, the subscriber unit may monitor paging messages or other overhead messages transmitted by nearby base stations in order to receive notification of an incoming call. As the subscriber unit moves through the service area, it decides which base station is the best base station to monitor for receiving these messages.
When the subscriber unit is in an active mode, wherein the subscriber unit has been assigned a traffic channel, the subscriber unit scans signals from neighboring base stations in order to report the characteristics of those signals so that a mobility manager may determine whether or not a better communication link can be established with an alternate base station.
The list of base station signals scanned by the subscriber unit is contained in a neighbor scan list. This neighbor scan list contains neighbor identifiers that instruct the subscriber unit how to receive and measure signals from neighboring base stations. For example, in a CDMA (Code Division Multiple Access) system, a neighbor identifier may include a frequency, a pilot offset, and other information describing the configuration of that base station. Such configuration information may describe the number and location of the paging channels for the base station.
The subscriber unit typically receives a neighbor scan list as one of the overhead messages that is transmitted on the paging channel to all subscribers currently monitoring that paging channel. After receiving a neighbor scan list from the paging channel, the subscriber unit typically enters the idle mode, wherein the subscriber unit is waiting to receive or initiate a call.
One problem with the neighbor scan lists that are broadcast on the paging channel is that these lists are often too long for idle mode scanning. They are too long because they contain neighbor identifiers that are needed only after the subscriber enters the active mode. The problem with scanning too many neighbors in the idle mode is that it reduces standby battery life by causing the subscriber unit to stay awake longer to scan signals associated with all the neighbor identifiers.
Additionally, the subscriber unit may be asked to scan signals from base stations that do not support a traffic channel. For example, some cells may have a base station that transmits a pilot channel, and does not support a traffic channel. These cells are often referred to as a "pilot beacon cells." These pilot beacon cells are used to determine that a subscriber unit is reaching the outer limits of the cellular coverage area. In some instances, the subscriber unit may attempt an idle-mode handoff to one of these "pilot beacon cells" with no traffic channel. This may result in additional delays in acquiring a new cell that supports traffic channels. These delays occur because the handoff is not direct to a cell that supports a traffic channel--rather the handoff is indirect to a pilot beacon cell that expressly, or by default, directs the subscriber unit to a cell on another frequency. During this "indirect idle-mode handoff" the subscriber unit may not be able to receive an incoming call or originate an outgoing call.
After a call has been received or initiated, and after the subscriber unit has been assigned a traffic channel, the subscriber unit may receive messages that update the neighbor scan list while it is in the active mode. But there are problems with receiving neighbor scan list updates when the subscriber unit is in the active mode.
First, sending neighbor scan list update messages must be directed to a particular user, and second, the list must be transmitted in the CDMA traffic channel. When in the idle mode, neighbor scan lists are broadcast to all listening subscriber units, which requires less message processing and does not displace any data in the traffic channel. In known cellular systems, a lengthy message may be required to modify the neighbor scan list or transmit a totally new neighbor scan list. Such lengthy messages in the traffic channel may degrade voice or user data transmission.
With reference now to FIG. 1, there is depicted a portion of a service area 20 of a cellular communication system. Shown in service area 20 are fifteen cells numbered C0-C14. Cells C0-C4 each contain two carriers operating on frequencies F.sub.1 and F.sub.2. For example, as shown in cell C3, the two carrier frequencies are shown at reference numerals 22 and 24. As indicated by solid lines, both frequency F.sub.1, 22 and F.sub.2, 24 support a traffic channel, which transmits user data.
Cells C5, C6 and C8 each contain a traffic channel on frequency F.sub.1. However, on frequency F.sub.2, cells C5, C6 and C8 have a pilot beacon, and perhaps a paging channel and a sync channel, but no traffic channel. The pilot beacon on frequency F.sub.2 is shown at reference numeral 26 with a dashed line.
Cells C7, C9-C11 only broadcast on frequency F.sub.1 ; these cells do not have any transmission on frequency F.sub.2. Cells C12-C14 have only a pilot beacon on F.sub.1, as indicated by dashed lines.
The cell configurations shown in service area 20 analogously resemble a wedding cake, which may occur at the boundary between a high traffic area and a lower traffic area, wherein the high traffic area is supported by two carrier frequencies. On the boundary between the high traffic area and the lower traffic area, pilot beacons are used to help the mobility manager decide when a call should be handed off from frequency F.sub.2 to frequency F.sub.1.
When subscriber unit 28 powers up in cell C0 on F.sub.2, the F.sub.2 paging channel transmits neighbor scan list 30 to all subscriber units monitoring the paging channel. Note that when the subscriber unit is in the idle mode, it can make measurements on more than one frequency because it can retune its receiver without having to worry about continuously receiving traffic. Thus, neighbor scan list 30 includes neighbor identifiers for cells C1-C7 on frequency 1, and also neighbor identifiers for cells C1-C6 for frequency 2.
The neighbor identifiers that comprise neighbor scan list 30 are typically selected by the cellular communications system designer because they are signals associated with base stations with which the subscriber unit is most likely to establish its next communication link. For example, as subscriber unit 28 moves out of cell C0 it is likely that one of the signals identified in neighbor scan list 30 will become a more desirable signal with which the subscriber unit can maintain communication with the cellular communication system. Neighbor scan list 30 merely predicts and informs the subscriber unit where its next communication link with the communication system may be established rather than relying upon the subscriber unit to find the next base station link by itself.
In some situations, neighbor scan list 30 includes neighbor identifiers, such as C5 F.sub.2 and C6 F.sub.2, which are pilot beacon cells. Including pilot beacon cells in the neighbor scan list causes the problem associated with the indirect idle-mode handoff, as discussed above.
The reason pilot beacons are included in neighbor scan list 30 is because the subscriber unit may use them after entering the active mode to perform a hard handoff from, for example, frequency F.sub.2 to frequency F.sub.1 in cells C5, C6, or C8. A problem arises when subscriber unit 28 initiates an outgoing call just as subscriber unit 28 travels out of cell C0 while communicating on frequency F.sub.2. To facilitate a subscriber handoff to frequency F.sub.1, subscriber unit 28 reports handoff information obtained from receiving pilot beacons and paging channels on F.sub.2 from cells C5, C6, and C8. If this handoff information is not immediately available upon entering the active mode as the subscriber leaves C0, the call may be dropped. Therefore, in the prior art, the neighbor can list used in the idle mode included extra neighbor identifiers that were needed only after the subscriber entered the active mode. This made the neighbor scan list used in the idle mode longer than it needed to be for idle mode operation.
Thus, it should be apparent that there are disadvantages to having to many or to few neighbor identifiers in the neighbor scan list as the subscriber unit changes between an idle mode and an active mode. Therefore, a need exists for an improved method and system for controlling neighbor scanning in a subscriber unit in a cellular communication system.